Role of charge transfer for the vibrational-mode-specific chemical reaction of excited ammonia(1+) cation and ammonia

Abstract

New reaction pathways involving (NH[sub 3] [center dot] NH[sub 3])[sup +] charge-transfer (CT) complex formation have been proposed for the vibrational-mode-specific reaction of NH[sub 3][sup +]([nu]) with NH[sub 3] using the concept of a meta-IRC (intrinsic reaction coordinate). It is known experimentally for the reaction that there are three elementary processes, electron transfer (ET), proton transfer (PT), and hydrogen abstraction (HA). The CT complex formation plays a key role in the authors' proposed reaction pathways for both ET and HA processes: in ET the reactants form the CT complex, which then dissociates to NH[sub 3][sup +] and NH[sub 3] with an electron transferred from one to the other, while in HA the CT complex does not dissociate but undergoes a rearrangement to form the (NH[sub 4] [center dot] NH[sub 2])[sup +], complex, which dissociates to NH[sub 4][sup +] and NH[sub 2]. The overall HA process has been predicted to have no energy barrier and to be exothermic by 15.8 kcal/mol at the HF/4-31G level of calculation. The authors have suggested that three characteristic transition points (TP's) exist for the reaction pathway of HA: (1) the primary TP of the CT process prerequisite for the HA to occur, (2) the geographical more » TP of the potential surface, i.e., the conventional transition state (TS) for the HA process, and (3) the secondary TP of the geometrical change for the HA process. For the ordinary reaction coordinate, the conventional TS merges the TP of CT and the geometrical TP, but in this case the character of the conventional TS splits into the three TP's. In particular, it is found that the CT process around the primary TP is most significant for the vibrational-mode specificity of the ET and HA processes. Moreover, their mechanism elucidates the non-state-selected experimental results: the reaction proceeds via PT (85%) rather than HA (15%) and ET is negligible. « less